Batch injection stripping voltammetry of trace metals has been used before for the analysis of trace metals in the ppb range. Amongst others there has been described a non-flow approach for anodic stripping voltammetry (ASV) of small discrete samples, using a batch injection operation. There has also been described anodic stripping coulometry at a thin-film mercury electrode where the charge contained in the stripping signal is used as indicator of traces of metals.
A variety of microcells has been described for voltammetry and for coulometric measurements.
The main requirements for a microcell for such analysis are, amongst others:
a. Provision for a microvolume sample; PA1 b. Rotating electrode to decrease the detection limit; Batch injection of samples without interruption of the cathodic polarization of the working electrode, preventing mercury film oxidation on such electrode.
Microcells described in literature have certain drawbacks and are not entirely satisfactory.
The microcell-precursor (R. Eggli, Anal. Chim. Acta, 91 (1977) 129) consists of a microcompartment (100-600 mL)with a rotating working electrode, but this design does not allow the batch injection of samples. The use of such a microcell makes the analysis more complicated and time-consuming, and requires interruption of the working electrode cathodic polarization during replacement of a sample, which can cause oxidation of the mercury film on the working electrode. Using coulometric detection for anodic stripping voltammetry, the author has obtained a comparatively high detection limit. For lead it is 24-25 ppb at a deposition time equal to 12 min.
According to J. Wang et. al., Anal. Chim. Acta, 259 (1992) 123, a sample drop (100 mL) was put directly on a working electrode surface, which sample drop was surrounded by large volume of supporting electrolyte. The detection limit for anodic square-wave stripping voltammetry in this case was not too low due to the use of a static working electrode. For example, for lead the detection limit was equal to 1.2 ppb at a deposition time 0.5 min. At the same time batch injection of samples was used and the cathodic polarization of the working electrode was not interrupted. The sample throughput was 48 samples per hour. In spite of all precautions taken by the authors, the problem remained of how to avoid completely the mixing of a sample drop with the supporting electrolyte during the time when the drop was put into the cell and the deposition time.